1. Field of the Invention
The present invention relates to a method for manufacturing a semiconductor device.
2. Description of the Related Art
In a semiconductor product such as a liquid crystal display device drive integrated circuit (LDI), a source drive integrated circuit (IC) requires a dual gate process of a high voltage (HV) and a low voltage (LV), and a gate driver IC requires a triple gate process.
The dual gate process is divided into a process step for forming an HV device region and a process step for forming an LV device region.
To begin with, a first oxide layer is formed on an entire surface of a substrate to form an oxide layer in an HV device region. A portion of the first oxide layer formed in an LV device region is removed so as to form an oxide layer in the LV device region. A second oxide layer is formed thinner than the first oxide layer in the LV device region. After that, a process for forming a gate electrode and source/drain electrodes in respective device regions is performed.
The HV device region is formed by thermal oxidation of a first oxide layer to uniformly reduce a hump and improve a threshold voltage, Vt. And a portion of the first oxide layer formed in the LV device region is removed because a thinner oxide layer is required in the LV device region than in the HV device region.
However, the removing of the portion of the first oxide layer formed in the LV device region may cause damage to a substrate, particularly to a shallow trench isolation (STI), and thus it may cause a problem of a current leakage.
Further, when a process for forming a silicide in an HV and an LV device region is performed, an oxide layer has a different thickness in the HV device region than in the LV device region. The thickness difference may cause damage to the relatively thin LV device region in the substrate, and thus it may cause a current leakage in the LV device region.